1. Field of Invention
The invention relates to a voltage-controlled oscillator and, in particular, to a voltage-controlled oscillator having a bandpass filter.
2. Related Art
The voltage-controlled oscillator (VCO) is a frequency-varying oscillation circuit that changes its output frequency according to an external voltage. The varying range of the frequency is determined according to its purposes. It is a common element in high-frequency circuits or communication systems.
A conventional VCO shown in FIG. 1 includes a resonance circuit 100, an oscillation circuit 200, and a buffer amplifying circuit 300. The resonance circuit 100 is used to output a harmonic frequency signal which functions as the oscillation frequency of the oscillation circuit. The resonance circuit 100 includes capacitors C1, C2, C3 and C4, inductors L1 and L2, and a diode D1. When a tunable voltage Vt is imposed on the input terminal of the resonance circuit 100, it outputs a harmonic frequency signal accordingly. The oscillation circuit 200 outputs a first oscillation signal according to the harmonic frequency signal output from the resonance circuit 100. The oscillation circuit 200 is composed of a first transistor TR1 and a capacitor C7. The harmonic frequency signal is inputted into the base of the first transistor TR1, and the first oscillation signal is outputted from its emitter. The emitter of the first transistor TR1 is further connected in series with a resistor R4 and an inductor L3 functioning as the radio frequency choke. The buffer amplifying circuit 300 includes a second transistor TR2, a microstrip SLIN1 and a capacitor C8. The first oscillator signal is amplified into a first oscillation frequency through the buffer amplifying circuit 300. The first oscillation frequency is then outputted through the collector of the buffer amplifying circuit 300 and the capacitor C9. Furthermore, a capacitor C10 is coupled between the ground and the capacitor C9.
The resistors R1, R2, and R3 are used to determine the base bias voltages of the first transistor TR1 and the second transistor TR2. One end of the resistor R1 is coupled to a power supply Vcc, while the other coupled to the resistor R2. One end of the resistor R3 is coupled to the resistor R2, while the other to the ground.
Affected by the circuit elements and/or circuit board, the first oscillation frequency outputted from the VCO disclosed in FIG. 1 usually has some distortion. Such harmonic distortions (including second-order or higher harmonic distortions) render the circuit unable to achieve desired effects. The frequency of these harmonic distortions may even interfere other frequency ranges or even the same one. Therefore, how to reduce the harmonic distortion is an important concern in circuit designs.